The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Devices transforming electrical energy into a torque or transforming a torque into electrical energy operate with some efficiency. Some measure of the input is transformed into the output. However, in either operation, some measure of the input is lost, primarily as heat rejected to the device.
Known powertrain architectures include torque-generative devices, including internal combustion engines and electric machines, which transmit torque through a transmission device to an output member. One exemplary powertrain includes a two-mode, compound-split, electro-mechanical transmission which utilizes an input member for receiving motive torque from a prime mover power source, preferably an internal combustion engine, and an output member. The output member can be operatively connected to a driveline for a motor vehicle for transmitting tractive torque thereto. Electric machines, operative as motors or generators, generate a torque input to the transmission, independently of a torque input from the internal combustion engine. The electric machines may transform vehicle kinetic energy, transmitted through the vehicle driveline, to electrical energy that is storable in an electrical energy storage device. A control system monitors various inputs from the vehicle and the operator and provides operational control of the powertrain, including controlling transmission operating state and gear shifting, controlling the torque-generative devices, and regulating the electrical power interchange among the electrical energy storage device and the electric machines to manage outputs of the transmission, including torque and rotational speed. A hydraulic control system is known to provide pressurized hydraulic fluid for a number of functions throughout the powertrain.
Operation of the above devices within a hybrid powertrain vehicle require management of numerous torque bearing shafts or devices representing connections to the above mentioned engine, electrical machines, and driveline. Input torque from the engine and input torque from the electric machine or electric machines can be applied individually or cooperatively to provide output torque. Various control schemes and operational connections between the various aforementioned components of the hybrid drive system are known, and the control system must be able to engage to and disengage the various components from the transmission in order to perform the functions of the hybrid powertrain system. Engagement and disengagement are known to be accomplished within the transmission by employing selectively operable clutches.
Clutches are devices well known in the art for engaging and disengaging shafts including the management of rotational velocity and torque differences between the shafts. Clutches are known in a variety of designs and control methods. One known type of clutch is a mechanical clutch operating by separating or joining two connective surfaces, for instance, clutch plates, operating, when joined, to apply frictional torque to each other. One control method for operating such a mechanical clutch includes applying the hydraulic control system implementing fluidic pressures transmitted through hydraulic lines to exert or release clamping force between the two connective surfaces. Operated thusly, the clutch is not operated in a binary manner, but rather is capable of a range of engagement states, from fully disengaged, to synchronized but not engaged, to engaged but with only minimal clamping force, to engaged with some maximum clamping force. The clamping force available to be applied to the clutch determines how much reactive torque the clutch can carry before the clutch slips.
The hydraulic control system, as described above, utilizes lines filled with hydraulic fluid to selectively activate clutches within the transmission. However, the hydraulic control system is also known to perform a number of other functions in a hybrid powertrain. For example, as described above, an electric machine utilized within a hybrid powertrain generates heat. Known embodiments utilize hydraulic fluid from the hydraulic control system to cool the electric machine in a machine cooling function. Additionally, known embodiments utilize hydraulic fluid to lubricate mechanical devices, such as bearings. Also, hydraulic circuits are known to include some level of internal leakage.
Hydraulic fluid is known to be pressurized within a hydraulic control system with a pump. The pump can be electrically powered or preferably mechanically driven. In addition to this first main hydraulic pump, hydraulic control systems are known to also include an auxiliary hydraulic pump. The internal impelling mechanism rotates operates at some speed, drawing hydraulic fluid from a return line and pressurizing the hydraulic control system. The supply of hydraulic flow by the pump or pumps is affected by a number of factors, including but not limited to the speed of the pumps, the back pressure exerted by the hydraulic line pressure (PLINE), and the temperature of the hydraulic fluid (TOIL).
The resulting or net PLINE within the hydraulic control system is impacted by a number of factors. FIG. 1 schematically illustrates a model of factors impacting hydraulic flow in an exemplary hydraulic control system, in accordance with the present disclosure. As one having ordinary skill in the art will appreciate, conservation of mass explains that, in steady state, flow entering a system must equal the flow exiting from that system. As applied to FIG. 1, a flow of hydraulic oil is supplied to the hydraulic control system by the pumps. The flow exits the hydraulic control system through the various functions served by the hydraulic control system. This exemplary embodiment includes the following functions: hydraulic oil fills clutch mechanisms in order to provide clamping force required to lock the clutch, as described above; hydraulic oil provides cooling of the electric machines and other components as required; hydraulic oil is used to lubricate portions of the transmission; and hydraulic oil flows through leakage internal to the hydraulic circuit. PLINE describes the resulting charge of hydraulic oil maintained in the system: for any flow through a system, the resulting pressure within the system depends upon the flow resistance within the system. Higher flow resistance in the system results in higher system pressures for a given flow. Conversely, lower flow resistance in the system results in lower system pressures for a given flow. Applied to FIG. 1, PLINE or the pressure within the hydraulic control system, changes depending upon usage of the hydraulic control system. For example, filling a previously unfilled transmission clutch consumes a significant amount of flow from the hydraulic control system. The orifice leading to the clutch includes low resistance in order to draw the significant amount of hydraulic oil over a short time span. As a result, during the clutch filling process, PLINE in an otherwise unchanged hydraulic control system will reduce. Conversely, for a given set of functions served by the hydraulic control system, PLINE varies based upon the flow supplied by the pumps. For any given set of flow restrictions associated with the functions served, increased flow from the pumps will result in higher PLINE.
The lubrication function served by the hydraulic control system includes some flow of hydraulic oil to the electric machine or machines utilized by the hybrid powertrain, for example, to keep a bearing from failing due to frictional forces within the bearing. Such frictional forces and the resulting stresses and heat upon the bearing increase with the speed of the device driving the bearing. Bearings are designed to work with an adequate level of lubrication to reduce the frictional forces within the bearing. If such a bearing is starved of adequate lubrication, then forces, stresses, and heat within the bearing quickly deviate from designed levels.
Known control methods to lubricate devices within the hybrid powertrain include utilizing the hydraulic control system to provide a flow through a fixed orifice loop. In such an exemplary arrangement, the resulting flow to the coolant loop will increase proportionally to an increase in PLINE. Additionally, a pressure-sensitive valve is known to be utilized, wherein increasing PLINE opens the valve, lowering the flow resistance of the orifice created by the valve. As a result, flow will increase non-linearly with increasing PLINE, with extra flow resulting from an incremental increase in PLINE. Such mechanical valves are beneficial because no sensor is needed, and the primary sources of error in a pressure-sensitive valve, lag and hysteresis, generally will not significant deviation from flow levels required for lubrication.
As is well known in the art and as described above, lubrication required for bearings associated with operation of an electric machine increase as the rotational speed of the electric machine increases. However, as described above, the rate of hydraulic oil and, therefore, the flow of the hydraulic oil flowing to the device through a lubrication loop equipped with either a fixed orifice or a pressure-sensitive valve only increase only with PLINE. Because PLINE is frequently directly driven by an engine, and because electric machine operation is not tied to engine operation, PLINE will not necessarily increase as an electric machine speed increases. As a result, situations can occur where high electric machine speeds and low PLINE result in the electric machine not receiving sufficient lubrication. Such a condition can be avoided by designing the flow restriction of the coolant loop to provide sufficient lubrication for all foreseeable operating conditions of the electric machine, but such a design requires an excessive flow of hydraulic oil during periods when the lubrication requirements of the electric machine do not warrant the design. Additionally, valves are known to modulate flow based upon a control signal. Such control signals however are based upon sensor readings; are reactive to changes, for example, in temperature and generally include a lag time; require expensive sensors; and are subject to sensor failure. A method to control lubrication flow in a hydraulic control system by modulating PLINE based upon speeds of torque generative devices would be beneficial.